Researchers used databases, including PubMed, Web of Science, and Google Scholar, to conduct a comprehensive literature search strategy. Since our study focused on changes in foot morphology during child development, each search string had to contain the following key words, ‘Children’s feet’ and ‘Morphology’. Therefore, the following key phrases were retrieved using Boolean search syntax, [(Children’s foot) OR (Children’s feet)] AND [(morphology) OR (shape) OR (foot posture)] AND (age) AND (sex), with 134 documents found; [(Children foot) OR (Children feet)] AND [(morphology) OR (shape) OR (foot posture)] AND (foot (Children foot) OR (Children feet) AND [(morphology) OR (shape) OR (foot posture)] AND [(obesity) OR (overweight)], 112 searches [(Children foot) OR (Children feet)] AND [(morphology) OR (shape) OR (foot posture)] AND [(flat foot) OR (Pes cavus) OR (CMT) OR (Foot Valgus)]. Thus, a total of 207 articles were found. The search was not limited to the publication year. Studies released before January 2023 were included. A total of 453 uncensored duplicates were considered for topics, abstracts, and keywords. All copies were removed using reference software (Endnote) and manually checked by the Investigators (HJ, QM, and YG) before the literature screening.

The screening conditions followed the framework construction method of (Linares-Espinos et al., 2018) and the Preferred Reporting Items for Systematic Evaluation and Meta-Analysis (PRISMA) guidelines (Moher et al., 2009).

Inclusion: (1) the language of study is English; 2) young children, children, and adolescents; 0–1 years of age; sample size >1; 3) studies focus on changes in foot morphology; prospective or retrospective studies involved at least changes in foot biomechanics or biological features; abnormal foot type studies should be about the characteristics of morphological changes and injury risk, biological features, plantar pressure; 4) studies should focus on tracer changes over time.

Exclusions: 1) congenital foot disease; other diseases causing foot changes; 18 years of age or beyond; 2) foot has undergone surgery; 3) studies that (apparently) published duplicate results from the same subject sample as in previous publications obtained from the same group; biological feature values reported in the first study were excluded; 4) non-original articles (e.g., comments or conference articles); non-English articles; review; and 5) did not address foot biological features.

The preliminary search data was 453 articles, and after deleting duplicates, the search yielded 238 articles. Among them, there were 64 articles about sex and age changes in children’s feet. There were 52 articles on changes in the foot due to obesity and overweight. There were 122 articles on changes in specific foot types in children. After removing 100 articles from the keyword filtering in titles and abstracts; 32 studies with congenital foot disorders were excluded; 20 studies with other diseases causing foot changes were excluded; 9 studies with feet undergoing surgical treatment were excluded; 24 studies without foot biology were excluded; and 7 studies without foot morphology were excluded. Therefore, based on the screening eligibility criteria, a total of 32 studies were included for full-text screening. However, 3 studies did not meet the number of participants, 3 conference papers or reviews were excluded, and 1 study was retracted. Finally, 25 papers were found that satisfied the requirements for inclusion. Figure 1 illustrates the systematic search and selection process of the studies in detail.

The flattened MLA, showing that the foot type had smaller arch angle as well as greater CSI and SI factor volume, was the primary attribute of the flat foot. The typical z-value for the foot length was around −0.1 ⁵¹.

The findings from Table 2 showed that variations in skeletal architecture, foot posture, and plantar pressure distribution were influenced by differences in sex and age. These variations continued until maturity. Studies on infant foot posture have shown that boys often had flatter feet and lower arches than girls. With increasing age, 46.7% of children aged 8–10 years had neutral feet and 53.3% had medially rotated feet (Al Kaissi et al., 2016). 56.9% of children aged 11 to 13 had neutral feet, 39.7% had medially rotated feet, and 3.4% had posteriorly rotated feet (Al Kaissi et al., 2016).

2. Ball width is the x-y distance between points B1 and B2 projected; The distance between the instep and heel, measured horizontally, is known as the instep distance. Ball girth: the area around the forefoot that corresponds to the B1, B2, and BC points. (B2: fifth metatarsal; B1: first metatarsal; BC points: projected on the z-axis. This is shown in Figure 2.)

From the results in Table 3, (Mauch et al., 2008), concluded that flatfoot occurred at a higher rate in overweight children than in other normal weight and ultralight weight. Although a considerable number of studies have concluded that weight gain during growth can indirectly or directly cause flat feet, (e.g., plantar pressure due to weight gain and flattening of the MLA), the foot prosthesis could maintain the longitudinal arch through compensatory mechanisms (Aboelnasr et al., 2019). As the central nervous system matures in children, individuals would have better motor performance and balance. This would result in better control of lower limb posture (Swallen et al., 2005). Notably, the ossification of foot structures that accompanied skeletal development would allow the arch to remain stable under weight-bearing in children (athirgamanathan et al., 2019). External tibial rotation from the in-toe position at birth to the out-toe position during growth would result in a concomitant decrease in the morphology of the hindfoot exostosis (Atik and Ozyurek, 2014).

From the above analysis, it was clear that flatfoot may not be significantly correlated with changes in body mass index (Hawke et al., 2016; Aboelnasr et al., 2019). In other words, weight gain may not lead to flatfoot. (Hawke et al., 2016). found that children with greater internal rotation of the foot exhibited greater lower limb and overall body flexibility in a study of a sample of healthy asymptomatic children aged 7–15 years. Also, the findings of this study corroborated the discovery of a connection between flat feet and joint flexibility (Pfeiffer et al., 2006).

Foot arch deformities were rarely seen in the early childhood population (under 3 years of age). However, as children grow, the navicular bone, the final foot bone to ossify in children between the ages of two and five, was characterized by its fallibility and formative nature. As a result, it became a crucial consideration when evaluating the foot posture of four-year-old kids. Foot navicular ossification occurred later in boys than in girls, while the prevalence of pes cavus increased highly in boys between the period of 4 and 13 years, but the prevalence of pes cavus was frequent in the girl population (Alexander and Johnson, 1989; Aminian and Sangeorzan, 2008; Wozniacka et al., 2013; Chang et al., 2014).

Pes cavus was a common foot disorder in children while this disorder had an overall inversion of the foot, projection of the lateral edges of the foot, and inversion of the heel during standing (Wicart, 2012). This was generally caused by the deformity of pes cavus, which was cavus foot, a simple morphological feature but a normal variant often found in healthy individuals and growing children (Alexander and Johnson, 1989; Schwend and Drennan, 2003; Chang et al., 2014). While the other condition was direct cavus foot, which was the result of foot deformity and often only affected the sagittal plane (forefoot, hindfoot, or both) occurred only in the sagittal plane (forefoot, hindfoot, or both). Direct cavus foot may be associated with multiple causes, and several studies have shown that the condition was due to structural problems in the brain, spinal cord, peripheral nerves, or foot, while neurological disorders were seen primarily in the posterior cavus foot (Schwend and Drennan, 2003; Wicart, 2012). CMT was the most common neurologically caused disorder in this condition (Burns et al., 2009). The risk of deterioration in childhood can generally be averted with the right conservative care (orthotic realignment of the foot) (Wicart, 2012).

Changes in foot morphology in children were gradual over time, particularly the growth of foot length and width. The critical age for foot development was 6 years, and (Waseda et al., 2014). found that children’s foot length increased rapidly from the age of 6 years. Changes in foot morphology characteristics were minimal during the age interval of 10–11 years and stabilized at 12 years (Cheng et al., 1997; El et al., 2006; Catan et al., 2020). Cheng et al. (Cheng et al., 1997) found that children’s foot length and width increased by an average of 8–10 mm per year between the ages of 6–12 years. (Muller et al., 2012). reported that the length and width of the foot grew with age, it was demonstrated that the growth rate of foot length practically hit a plateau at 13 years for girls and 14 years for boys at that point (Evans et al., 2012; Waseda et al., 2014).

In terms of sex differences, the change in the foot was comparable, however, there could be variations between 8 and 10 years old (Delgado-Abellan et al., 2014). According to (Cheng et al., 1997), until the age of 3 years, variations in foot length and width in males were comparable to those in girls. Boys’ feet development increased after the age of 3 years. At the age of 9, boys and girls showed considerable disparities, according to research by Bosch, Gerss, and Rosenbaum (Bosch et al., 2010). Nevertheless, no information regarding foot length was provided in this investigation. (Stavlas et al., 2005). found that the average annual foot length growth rate was 4.3% for boys and 3.9% for girls. The study also revealed that the fastest growth rates were observed at ages 8-9 and 10-11 for boys, and ages 7-8 and 8-9 for girls. Thus, peak growth rates occur earlier in girls than in boys. The age of 7–12 years was the stage of rapid growth in foot arch height among Chinese boys and girls. Studies showed that the navicular height of boys’ feet would increase from 6 to 13 years of age (Szczepanowska-Wolowiec et al., 2021). The navicular or talar navicular joint line was often used to determine the height of medial longitudinal arch. Between the ages of 8 and 13, the arch height of girls rose. The arch height ratio in boys was practically flat until age 11 but considerably rose from age 11 to 13. The formula for the arch height ratio is AHR (%) = navicular height*100/foot length. Girls’ arch height ratios were largely flat until age 10 but considerably rose between age 10 and 12.

Foot morphology changed with growth and development. (Ana et al., 2017). performed the Foot Posture Index (FPI) test for both feet among 2,569 children aged 9–15 years during 2016–2018. The FPI was a clinical diagnostic tool designed to quantify the grade of foot position (posture), such as neutral, internally rotated, or posteriorly rotated. The index was developed using a simple six-factor method to assess foot morphology, to obtain simple and quantitative postures (Al Kaissi et al., 2016). It was reported that girls had a higher percentage of rotated feet than boys, and boys presented higher FPI-6 scores than girls, suggesting that boys had flatter feet and that children and adolescents aged 9–15 years had neutral or mildly internally rotated foot morphology. (Heidi and Priv, 2015). conducted a study on foot morphology in infants who just started walking independently. The study showed that boys’ feet were flatter and had a lower arch than girls’ feet, indicating that males were more likely than girls to have higher FPI-6 scores from an early age.

In other words, the effect of sex differences on foot morphology may be present since birth, and such a condition may exist during growth. The effect of age differences on the foot was also presented from the time of birth, and because of the age difference, the foot width and length grew at different rates, which was a dynamic process. (Al Kaissi et al., 2016). observed differences between children and adult populations, with children generally presenting a neutral foot and internally rotated foot stance, and (Wilkerson and Mason, 2000) found that adults generally presented a neutral foot stance. The study discovered that with increased muscle mass, myelination of motor neurons, and subsequently enhanced muscular strength from puberty forward, the medial longitudinal arch (MLA) had stronger support. This function would contribute to the development of a neutral foot.

In studies of normal foot type, characteristics of foot morphology could be evaluated by footprint, foot index, FPI, manual measurement, and camera acquisition. A large sample of data and studies reported (Alexander and Johnson, 1989; Aminian and Sangeorzan, 2008; Heidi and Priv, 2015; Hawke et al., 2016; Escalona-Marfil et al., 2022) that incorrect gait and walking postures and related external factors were the causes of foot disorders in children.

Apart from the above factors of age and sex, variations in body weight and height could also lead to changes in the biological structure of the foot (Waseda et al., 2014; Bailly et al., 2022). Being overweight has implications for foot structure, including alterations of anatomical structure, abnormalities in plantar pressure distribution, and balance (Bruner et al., 2009; Waseda et al., 2014; Manousaki et al., 2019; Bailly et al., 2022). BMI is a popular tool for assessing overweight and obesity as a straightforward measure of the connection between weight and height. The overweight individuals have higher ratios of arch collapse due to thicker layers of adipose tissue (Swallen et al., 2005). Medial section of the plantar foot in a group of 8-year-old children had denser adipose tissue, and the height of longitudinal arch was lower in these observations (Wilkerson and Mason, 2000). (Agnieszka and Edyta, 2015) found that being overweight had a negative effect on the longitudinal arch in preschool children. In preschoolers, no correlation between BMI and the height of the transverse arch was found, but in preschoolers of normal weight, the height of longitudinal arch was significantly increased. In contrast, a trend towards the decreasing longitudinal arch height in overweight and obese boys and girls was found. (da Rocha et al., 2014). concluded that foot sensitivity was lower in obese children than non-obese children. Obese children showed similar sensitivity in all foot regions, while non-obese children were able to differentiate the intensities of touch on different regions. Studies showed that higher sensitivity to touch was associated with higher receptor density, which decreased with age. Therefore, the lower sensitivity in obese children may be due to lower receptor density per surface area unit (Kozłowska, 1988), but this is only theory at this time, and no particular experimental verification of this assumption has been carried out. Future studies may consider examining the receptor density of foot surface area in obese, overweight, normal, and ultralight children as per age group and BMI groups. (Lee et al., 2013).

Overweight boys during childhood may have bigger feet, indicating that obesity may be a major factor affecting foot growth (as determined by the size recorded in the available data set). The structural characteristics of the foot in obese children were wider and thicker, which may increase the peak plantar pressure and vertical peak pressure during gait. Lower footprint angle (FA), higher Chippaux-Smirak index (CSI), higher plantar pressure (Lobstein and Frelut, 2003), higher arch index (AI), and greater footprint area (Stovitz et al., 2008) were found in obese children feet. Children with high body weight may show less degree of variations in the foot morphology or the changes might be gradual or subtle. Thus, (Jimenez-Ormeno et al., 2013), suggested that being overweight may be an important factor affecting foot development in prepubescent students at school. (Mauch et al., 2008). found that low percentage of flatfoot was observed in underweight children. However, the likelihood of flat feet rises with age, although this is highly documented in overweight children.

Obesity would lead to the compensation of foot arch, and either indirectly or directly contribute to the development of flat feet (Sachithanandam and Joseph, 1995; Chen et al., 2013; Song-Hua et al., 2017; Buldt et al., 2018; Telfer and Bigham, 2019). However, a recent study of 728 children between 3 and 15 years old found a relationship between weight and foot morphology (Evans and Karimi, 2015). While Evans (Evans and Karimi, 2015) refuted that heavier children had flatter feet, and further emphasized no association between increased body weight and flatfeet among children. Whilst this statement contradicted with several studies (Sachithanandam and Joseph, 1995; Lobstein and Frelut, 2003; Chen et al., 2013; Telfer and Bigham, 2019), which may be a convergent prediction of the foot morphology in overweight and obese children. Currently, weight reduction was not employed as a treatment in the traditional treatment of flat feet due to obesity. Therefore, it was suggested that further follow-up studies should be conducted focusing on this issue. Yet, alterations in foot morphology brought on by excess weight may cause discomfort or pain and may increase reluctance to exercise, which might result in body weight growth. Considering the development of healthy children, increased attention is still required, especially for physical activity and diet control.